The traditional reliability of telecommunication systems that users have come to expect and rely upon is based in part on the systems' operation on redundant equipment and power supplies. Telecommunication systems, for example, route tens of thousands of calls per second. The failure of such systems, due to either equipment breakdown or loss of power, is unacceptable since it would result in a loss of millions of telephone calls and a corresponding loss of revenue.
Power plants, such as battery plants, address the power loss problem by providing the system with a secondary source of power, such as batteries, in the event of the loss of a primary source of power. A typical power plant includes a number of power converters, coupled in parallel, that provide power to operate the load and to charge the batteries. During normal operations, the power converters operate in a constant voltage mode to supply power to the load. The presence of a heavy load or a partially discharged battery, however, will lower the output voltage and require the power converters to switch to a constant current mode of operation.
Traditionally, a two loop controller has been used to control the power converters. A power converter employing the two loop controller typically includes a voltage loop and a current loop, coupled to a pulse width modulator (PWM). The power converter further includes a power stage, coupled to the modulator, that generates output power based on PWM signals received therefrom. The voltage loop and the current loop thus alternatively cooperate with the modulator to regulate the output power produced by the power stage.
The voltage loop includes a voltage mode controller, typically an operational amplifier (op-amp) and its associated components, that receives a feedback signal, such as an output voltage signal, from an output of the power converter. The op-amp compares the feedback signal to a reference signal and produces therefrom a control signal, such as a voltage error signal. The modulator receives the control signal from the voltage mode controller and adjusts the output of the power stage accordingly, thereby maintaining a substantially constant voltage output.
The current loop includes a current mode controller, typically an op-amp and its associated components, that receives a feedback signal, such as an output current signal, from the output of the power converter. The op-amp compares the feedback signal to a reference signal and produces therefrom a control signal, such as a current error signal. The current mode controller then sends the control signal to the modulator, which uses the control signal to adjust the output of the power stage to thereby maintain a substantially constant current output.
In a typical implementation of the two loop controller, the voltage and current loops operate in an "exclusive OR" fashion whereby only one controller (either the voltage mode controller or the current mode controller) is operational at any given time. The control signals from the voltage mode controller and the current mode controller, therefore, are usually voltage signals that are diode-coupled to the input of the modulator. Diode coupling allows the voltage and current mode controllers to dynamically acquire control of the modulator depending on the condition of the load. A major disadvantage of the two loop controller, however, is the saturation of the op-amp associated with the non-operational controller. For example, if the voltage mode controller is in control of the modulator, the current mode controller op-amp is in saturation and its output remains at a rail voltage. Therefore, if a transition between voltage mode and current mode is required, the current mode controller op-amp must ramp its output from the rail voltage to a proper operational voltage. An unacceptable period of time may pass before the output of the current mode controller op-amp reaches the required operational voltage. During this time period, the output voltage of the power converter is not regulated and may exhibit large overshoots or undershoots that may cause failures in the load equipment.
U.S. Pat. No. 5,731,692, entitled "System and Method for Limiting Overshoot in a Voltage and Current Control Circuit," issued to Garcia, et al. on Mar. 24, 1998, describes a control circuit for controlling one of at least two controllable characteristics of a controlled circuit. The aforementioned reference is incorporated herein by reference. Garcia discloses a control circuit employable to regulate the output voltage or output current of a power converter. The control circuit has an overshoot limiting circuit that prevents saturation of the non-operational error amplifier. The overshoot limiting circuit, however, increases circuit complexity and requires a significant amount of components and board real-estate.
Accordingly, what is needed in the art is a controller for DC-DC converters that, while reducing board real-estate, avoids saturation of the non-operational op-amp and provides a smooth transition between the voltage and current modes of operation.